1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones having an ensemble effect.
2. Description of the Prior Art
It is well known that the tonal quality of an electronic musical instrument is enhanced by generating tones which have an ensemble quality. The usual method of producing an ensemble effect is to generate two or more tones whose fundamental frequencies differ by some small frequency difference. The motivation for this arrangement of frequencies is to replicate the ensemble tone produced by a chorus of musical instruments which are not precisely in tune. It is this attribute of "out-of-tune" ensemble that produces the warm tone characteristic of a group of violins even when they are played in unison.
Various arrangements have been employed to electronically generage two simultaneous tones which are slightly detuned with respect to each other. The straightforward arrangement is to simply duplicate each tone generator and to use detuned clocks to drive the tone generators. This arrangement is plagued by the practical problem of implementing two independent clocks which must be only slightly out-of-tune and cannot be permitted to drift independently in frequency with changes in their ambient conditions.
In U.S. Pat. No. 3,809,792 entitled "Production Of Celeste In A Computer Organ" a system is described for producing a celeste version of ensemble tone by computing the amplitudes at successive sample points of a musical waveshape and converting the amplitudes to musical tones as the computations are carried out in real time. Each amplitude point is computed during the regular time interval by individually calculating and combining at least two sets of discrete Fourier components. The first set includes harmonically related components, generally the true pitch fundamental frequency and overtones of each keyboard selected note. The components of the second set are generated to be offset slightly higher in frequency than those in the first set.
In U.S. Pat. No. 4,112,803 entitled "Ensemble And Anharmonic Generation In A Polyphonic Tone Synthesizer" apparatus is disclosed for producing an ensemble effect in a polyphonic tone synthesizer of the type wherein musical tones are produced polyphonically by computing a master data set, transferring the data set to buffer memories, and repetitively converting in real time the contents of the buffer memories, and repetitively converting in real time the contents of the buffer memories to analog musical waveshapes. A multiplicity of master data sets are created repetitively and independently of tone generation by computing a generalized Fourier-type algorithm using stored sets of harmonic coefficients. The phase of these master data sets are generated with time varying phase shifts to produce the out-of-tune ensemble tone effect. The phase shifted master data sets are combined and transferred to the buffer memories.
In U.S. Pat. No. 4,205,580 entitled "Ensemble Effect In An Electronic Musical Instrument" apparatus is disclosed for producing an ensemble effect in a polyphonic tone synthesizer by providing a master data set of words having values corresponding to the relative amplitudes of equally spaced points along one cycle of the waveform of a musical tone. These values are transferred sequentially during the repetitive cycles at a rate proportional to the pith of the desired musical tone to a digital-to-analog converter for converting the master data set to an audio signal of the desired waveform and pitch. The ensemble effect is produced by transferring the words of the master data set to a second converter at the same clock rate but having one data point deleted or repeated once in the second set.
In U.S. Pat. No. 4,353,279 entitled "Apparatus For Producing Ensemble Tone In An Electronic Musical Instrument" a system is disclosed for producing an ensemble effect in a digital tone generator by providing a master data set of words having values corresponding to the relative amplitudes of equally spaced points along one cycle of a waveform of a musical tone in which the fundamental frequency has been deleted. These values are read sequentially and repetitively from a memory to produce a first analog tone. A second analog tone is produced by multiplying a data set corresponding to the fundamental frequency by a low frequency sinusoid waveshape set of points. The first and second analog tones are summed to yield a musical tone having an ensemble effect.
In U.S. Pat. No. 4,476,691 entitled "Selectable Ensemble Effect In An Electronic Musical Instrument" a system is described for producing an ensemble effect in a digital tone generator by implementing a plurality of tone generators such that each has an ensemble tone effect created by adding together a number of composite tones. The tone generation is produced by sequentially and repetitively accessing a memory containing a set of data points which define a period of a preselected musical waveshape. Each tone generator is implemented by selected a corresponding set of data points read out of memory. The selection logic is controlled by select gates and a comparator which is responsive to the frequency of an associated actuated keyboard switch.